Some types of implantable medical devices are used in conjunction with one or more implantable leads. For example, a cardiac rhythm management device (e.g., a pacemaker, a defibrillator, or a cardioverter defibrillator) implanted at a subcutaneous site may connect to one or more leads that terminate at or near the heart of a patient. Through the use of such a lead, the device is able to effectively monitor cardiac function and provide stimulation therapy for a patient who suffers from cardiac arrhythmia.
If a patient with a lead-based implantable medical device is subjected to magnetic resonance imaging (“MRI”) scanning, the device could malfunction in some cases. For example, time-varying magnetic fields generated during an MRI scan may induce current in an implanted lead. A relatively large current and/or voltage may be generated at the implantable medical device as a result of this induced lead current. In some cases, this current and/or voltage may affect the internal circuitry of the implantable medical device and cause the device to malfunction.
In an attempt to address such MRI issues, implantable medical devices may incorporate an MRI radiofrequency (RF) filter that is tuned to filter out strong MRI-induced RF signals (e.g., in the range of 64 MHz and 128 MHz). Such a filter may be incorporated into an implantable lead and/or in an implantable device.
In practice, however, these MRI RF filters may not adequately filter MRI gradient signals (e.g., depending on the manufacturer and MRI scanner model, MRI gradient signals may have a major bandwidth from hundreds of hertz up to several hundred kilohertz). Of note here, the MRI gradient signal frequency range is relatively close to the frequency range of sensed cardiac signals (e.g., 100 Hz or less). Moreover, the amplitude of the MRI gradient signal may be much larger than the magnitude of a sensed cardiac signal. Thus, the MRI gradient signal may swamp the receive amplifiers of the implantable medical device in some cases, thereby preventing the implantable medical device from being able to accurately detect other incoming signals (e.g., cardiac signals). Hence, it is relatively difficult to design a filter that efficiently rejects the MRI gradient signal and allows the desired cardiac signals to pass in all cases. Accordingly, there is a need for a technique that effectively rejects MRI gradient signals in implantable medical devices.